An ultrasonic motor generally has a construction such as that described in Patent Reference 1, below. Hereafter, this construction is explained, with reference to the rough cross-sectional diagram shown in FIG. 7. In this example, stator 52 and rotor 53 are constructed such as to be pressurized by spring 54 and appressed. The surface of stator 52 which is in contact with rotor 53 is formed as a pectinate, and piezoelectric ceramic 51 is provided on the surface on the opposite side thereof. In a construction as such, a traveling wave is generated in piezoelectric ceramic 51 when an ultrasonic wave is impressed thereto, and this traveling wave is transmitted to rotor 53 via the pectinate surface and the rotor 53 rotates in the direction of the traveling wave (straight forward movement in the case of a linear motor). Although, the zoom lens of a camera is common as a use of the ultrasonic motor, the rotor 53 is rotationally-driven at a comparatively high speed (for example, 40 rpm) when utilized for such uses.
Patent Reference 1: Japanese Patent Application Publication H2-261077
In conventional three-dimensional ultrasonic diagnostic devices, generally, a three-dimensional image with three directions of the arc direction of a sonic element, the rotational scanning direction and the depth direction are obtained by performing scanning with the arc shaped sonic element rotated in a direction perpendicular to the arc direction. In addition, although, in recent years, the ultrasonic motor has been utilized as a driving means for rotating sonic elements, the rotor 53 is rotationally-driven at a comparatively low speed (for example, 10 rpm or less) when utilized for such uses.
However, when such a conventional ultrasonic motor is driven for a long period of time at a relatively low speed, there occur problems such that roughness occurs on the surface of the pectinate stator 52 at the side which is in contact with the rotor 53, and also, fine particles of the stator 52 are transferred to the contact surface of the rotor 53 so that stable operation will not be obtained, resulting in a short life. Furthermore, in an ultrasonic diagnostic device utilizing this ultrasonic motor, a three-dimensional image cannot be obtained because stable operation can no longer be attained.